In molten metal processing, troughs are generally used to convey the molten metal from the melting furnace to various process devices, such as casting moulds, analyzers, etc. Where there is ample metal flow, there is sufficient sensible heat in the molten metal to compensate for heat losses via the trough. However, in low metal flow situations, or long trough runs, some form of trough heating is required to prevent excessive metal heat loss.
U.S. Pat. No. 3,494,410 (Birchill et al) discloses a generic heated trough with provision for heating from any side, but without any detail as to how to achieve efficient thermal control.
U.S. Pat. No. 4,345,743 (Sivilotti) teaches an adiabatic tubular discharge conduit including a refractory lining for containing the molten metal and encompassed by heater elements.
U.S. Pat. No. 6,444,165 (Eckert) dated Sep. 3, 2002, describes a heated trough having heaters embedded in the side walls or bottom, in close contact with the refractory material of the trough.
These two references both use heaters which heat by conduction. Such heaters tend to form hot spots and uneven heating due to the difficulty in ensuring good contact with the surrounding refractory. They can be difficult to maintain because of a tendency get stuck by metal impregnation or expansion of the element shell. With heaters operating by thermal contact, the temperature difference between heater and surrounding materials will be low in the case of good thermal contact, and therefore a high heater temperature is needed to achieve a good watt density and energy transfer.
U.S. Pat. No. 4,531,717 (Hebrant) discloses a covered heated trough with heaters mounted in the top cover. These heaters operate principally by radiation rather than conduction and the heat flux is dependent on a fourth power of the temperature of the heater and the surface receiving the radiation. Radiative heaters are capable of high watt densities. However, heaters radiating to the surface of a molten metal are overall inefficient because of the low emissivity of the surface.
There is a common tendency for impurities, or inclusions, to form in molten metal as it travels down the trough, which is made more serious for troughs heated from above (top-heated). These inclusions grow in the presence of atmospheric oxygen at the surface of the trough, and refractory materials from which the troughs are usually made. High temperatures provided at the molten metal surface and low metal flow rates increase this effect dramatically.
It is therefore desirable to find a trough heater arrangement that will provide even and controllable heat to the traveling molten metal, while reducing formation of inclusions.